Abstract
Previous studies have shown that numerous sprouts originating from a neuroma, after nerve injury in neonatal animals, can invade spinal nerve roots. However, no study with a focus on how such sprouts behave when they reach the border between the central and peripheral nervous system (CNS–PNS border) has been published. In this study we have in detail examined the CNS–PNS border of ventral roots in kittens with light and electron microscopy after early postnatal sciatic nerve resection. A transient ingrowth of substance P positive axons was observed into the CNS, but no spouts remained 6 weeks after the injury. Using serial sections and electron microscopy it was possible to identify small bundles of unmyelinated axons that penetrated from the root fascicles for a short distance into the CNS. These axons ended blindly, sometimes with a growth cone-like terminal swelling filled with vesicles. The axon bundles were accompanied by p75 positive cells in both the root fascicles and the pia mater, but not in the CNS. It may thus be suggested that neurotrophin presenting p75 positive cells could facilitate axonal growth into the pia mater and that the lack of such cells in the CNS compartment might contribute to the failure of growth into the CNS. A maldevelopment of myelin sheaths at the CNS–PNS border of motor axons was observed and it seems possible that this could have consequences for the propagation of action potential across this region after neonatal nerve injury. Thus, in this first detailed study on the behavior of recurrent sprouts at the CNS–PNS border.
Highlights
In pioneering studies it was shown by Coggeshall and co-workers that ventral roots of cats and man contain a large proportion of unmyelinated sensory axons (Coggeshall et al, 1974; Coggeshall, 1980), the ventral root afferents (VRA)
Light microscopy Compared to normal young cats, the content of substance P (SP) positive nerve fibers was markedly elevated in the juxtamedullary L7/S1 ventral roots ipsilateral to the neuroma in all operated kittens surviving longer than 14 days after the sciatic nerve lesion
Our previous studies have shown that ventral roots connected to sciatic neuromas may accommodate thousands of sprouts (Risling et al, 1984c) and that many of the sprouts probably originate from sensory axons (Risling et al, 1984a)
Summary
In pioneering studies it was shown by Coggeshall and co-workers that ventral roots of cats and man contain a large proportion of unmyelinated sensory axons (Coggeshall et al, 1974; Coggeshall, 1980), the ventral root afferents (VRA). At some levels the VRA comprise about 30% of the population; some 1500–2000 unmyelinated sensory axons are present in the ventral root L7 of adult cats (Coggeshall et al, 1974; Risling et al, 1981; Nilsson Remahl et al, 2008). The VRA have been implicated to carry sensory information from the pia mater and to be the background for the pain sensitivity of the ventral roots that was described by Magendie and in clinical studies (Frykholm et al, 1953; White and Sweet,1955; Cranefield,1974).,VRA could possibly be described as a type of nervi nervorum, an innervation of the nerve tissue, blood vessels or the surrounding connective tissue, and could exert a role for pain transmission in response to disc herniation or other spinal injuries. Most of them seem to have their cell bodies in the dorsal root ganglion, some may be attributed to aberrant sensory neurons in ventral roots (Risling et al, 1994)
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